1. Field of the Invention
The present invention generally relates to a method of producing plate making masks and a mask forming film. It particularly relates to a method of producing plate making masks useful in multicolor printing or the like in the field of direct plate making. It further relates to a mask forming film suitable for producing the masks.
2. Background of the Invention
A technique for producing a printing plate by utilizing an electrophotographic process or a silver salt diffusion transfer process is being popularized because of the simplicity of the technique. The popularization of the technique causes an increase in the requests for the production of multicolor printed matter or printed matter in which tone is partially designated through such a direct plate making process as described above. In this case, to cope with the color separation or screening in accordance with the designation, it is necessary to produce masks for covering designated portions of a press-ready copy.
For example, when dichromatic printing is performed, the following plate making process has been employed for a press-ready copy having a simple pattern. According to the process generally used, white paper is cut so as to form a mask for covering a portion of the press-ready copy unnecessary for the first color.
An electrophotographic printing plate is exposed to light reflected from the masked press-ready copy and is then developed to obtain a printing plate for the first color. Next, similarly to this, another portion of the press-ready copy unnecessary for the second color is covered with another mask similarly prepared. Another electrophotographic printing plate is exposed to light reflected from the masked press-ready copy and then developed to obtain another printing plate for the second color.
In the case where it is necessary to perform color separation in fine detail, on the other hand, the following plate making process has been employed. In this process, a press-ready copy is duplicated so that the thus obtained first and second press-ready copies are used respectively for the first and second colors. In each of the press-ready copies, an unnecessary portion is painted out with a white correction liquid or the like. Next, printing plates for the two colors are made of electrophotographic printing plates by using the two prepared press-ready copies.
In such conventional processes, however, there have been disadvantages in the following points. That is, the conventional process employing masks prepared by cutting white paper can be applied only to the case of a press-ready copy having a simple pattern. In making the covering masks, errors are apt to be caused between the first and second colors when mask-covered portions are formed. In the latter process in which color separation has to be made in fine detail, on the other hand, cost and time are taken for duplication of a press-ready copy, and the painting with a white correction liquid is a time-consuming work and may cause errors in printing. Moreover, it is difficult to carry out those conventional processes in the case where complicated color separation or screening for designated portions is performed. These disadvantages have limited the range of utilization of the direct plate making process.